Six anomalous vibronic feature states [~2 cm-1
full-width at half-maximum (FWHM), each consisting of ~20
partially resolved eigenstates] have been observed in
stimulated emission pumping (SEP) spectra of
C2D2. Of the two plausible
assigmnents for these features, the one most consistent
with spectroscopic observations would imply that the lowest
energy cis-bent triplet state of acetylene has
T0 <= 25820 cm-1, which is
inconsistent with previous ab initio predictions.
New higher level ab initio quantum mechanical methods have
been used to predict the energy difference between X
1Sigma+g ground state and the
cis-bent a 3B2 lowest triplet state
of acetylene. In conjunction with a triple zeta plus double
polarization plus f function (TZ2Pf) basis set, the coupled
cluster including single, double, and linearized triple
excitations CCSD(T) method yields T0 = Delta E(a
3B2 - X
1Sigma+g) = 30500
cm-1. The true value of T0 for the
3B2 state is estimated to be
approximately 500 cm-1 higher. At the same level
of theory the zero-point levels of the lowest triplet state
of the trans-bent (a 3Bu) and
vinylidene (a 3B2) isomers lie at
still higher energies. This result conclusively rules out
any triplet assignment for the anomalous feature states.
The alternative assignment, as highly excited vibrational
levels of the X 1Sigma+g
state, is surprising in view of the Franck-Condon
selectivity, dynamical stability, and nonselective
relaxation of this special class of ``bright states''
observed in the SEP spectra. Such an assignment would be
implausible in the absence of the present ab initio
calculations. Previous experimental observations [Lisy and
Klemperer, J. Chem. Phys.72, 3880 (1980)
and Wendt, Hippler, and Hunziker, J. Chem. Phys.70, 4044 (1979)] of acetylene triplet states are
discussed and shown to be completely consistent with each
other and with the present ab initio ordering of the cis
and trans isomeric minima on the T1 potential
energy surface: cis a 3B2 below trans
a 3Bu.